Hazard assessment and risk analysis of two new synthetic vitreous fibers

Isofrax and Insulfrax are two new synthetic vitreous fibers (SVFs) develope d for high-temperature insulation (1800-2300 degrees F) applications. In an attempt to significantly reduce or eliminate the potential of adverse heal th effects, these two fibers were specifically designed to have high solubi lity and, thus, low in vivo biodurability. In this paper, we review the eff ects of chemical composition on biodurability, in vitro fiber dissolution r ates (K-dis), and the relevance and relationship of K-dis to pulmonary fibr osis and lung tumors in chronic rat inhalation studies. We also examine the correlations between K-dis and weighted in vivo half-life (t(0.5)) of long fibers (>20 mu m) and their relation to pulmonary effects in chronic rat i nhalation bioassays. Predictions for outcomes of inhalation bioassays and d evelopment of nonsignificant risk levels of exposure are provided Additiona lly, justification for the use of inhalation versus noninhalation animal da ta is provided as is a brief review of human health effects of SVFs. me con clude, infer alia, that Isofrax and Insulfrax have low biodurability, would not be expected to produce either pulmonary fibrosis or lung tumors in a w ell-designed animal inhalation bioassay, have weighted half-lives beneath t he threshold established by the European Union for classification as a carc inogen, and based on epidemiological data for SVFs would not be expected to result in incremental cancer in human cohorts. Finally,it is estimated tha t approximately 90% of workplace exposure concentrations of these materials would be beneath 1 f/cc. At a concentration of 1 f/cc, neither fiber would be expected to result in an incremental working Lifetime cancer risk great er than 10(-5). (C) 1999 Academic Press.